Presently, magnetic recording media are commercially available which have various magnetic characteristics. Higher recording densities are the properties required of magnetic media of the next generation. Magnetic powders of higher coercive force are required to fulfill this requirement. The coercive force of magnetic powders is dependent largely on magnetic shape anisotropy attributable to their shape and magnetic crystalline anisotropy attributable to their magnetic energy. The present invention relates to a method of giving an improved coercive force by enhanced shape anisotropy. More particularly, the invention relates to a method of obtaining a high coercive force by using fine particles having a great axial ratio and utilizing the shape thereof for preparing a magnetic powder which is excellent in acicular properties.
Fine particles having an increased axial ratio generally deform and sinter under a thermal load during preparation, encountering difficulty in achieving the contemplated purpose.
Conventionally, fine particles are coated with aluminum, silica or like sintering preventing agent and are thereby prevented from deforming or sintering (e.g., U.S. Pat. No. 4,956,220). However, particulate materials, when in the form of fine particles, are liable to sinter during reaction and therefore require a large amount of sintering preventing agent, which increases the amount of nonmagnetic component and hampers the reduction reaction to result in the drawback of impaired magnetic characteristics.
An object of the present invention is to provide an acicular fine particulate material which is prepared by a promoted reduction reaction while being effectively prevented from sintering and which consequently affords magnetic recording media having a high recording density, and to provide a process for preparing the particulate material, and magnetic coating compositions and magnetic recording media containing the material.